V.generalize tutorial - Revision history

Hamish: /* Simplification */ add some explaining

2026-03-18T05:02:48Z

Simplification: add some explaining

← Older revision		Revision as of 05:02, 18 March 2026
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	</strike>		</strike>

	Douglas-Peucker Algorithm has very reasonable results, but it is very hard to find(guess) the		Douglas-Peucker Algorithm has very reasonable results, but it is very hard to find (i.e. guess) the
	~~right~~ value of threshold. Moreover, it is also impossible to simplify each line to (for example) 40%.		correct value to use for ''threshold=''. Moreover, it is also impossible to simplify each line to (for example) 40%.
	Exactly for such cases, ''v.generalize'' provides method=douglas_reduction. This algorithm		Exactly for such cases, ''v.generalize'' provides method=douglas_reduction. This algorithm
	is a modification of Douglas-Peucker Algorithm which takes another ~~paratemer~~ ''reduction''		is a modification of Douglas-Peucker Algorithm which takes another parameter ''reduction''
	which denotes (approximate) simplification of lines. For example,		which denotes (approximate, +/- a few percent) simplification of lines. A value of ''reduction=10'' will keep (approximately) 10% of the original vertices and drop the remaining 90%.

			For example,

	<pre>		<pre>

Neteler: remove hardcoded version specific urls, use latest version

2018-12-04T10:32:10Z

remove hardcoded version specific urls, use latest version

← Older revision		Revision as of 10:32, 4 December 2018
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	== MODULE ==		== MODULE ==

	* GRASS 6.4 {{cmd\|v.generalize}}		* GRASS 7.x {{cmd\|v.generalize}}
	* GRASS 7.x {{cmd\|v.generalize~~\|version=70~~}}

	== TUTORIAL ==		== TUTORIAL ==

⚠️Mmetz: + layer=1 for cats and where options

2018-11-13T15:53:39Z

+ layer=1 for cats and where options

← Older revision		Revision as of 15:53, 13 November 2018
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	<pre>		<pre>
	v.generalize -c input=roads output=roads_douglas_reduction2 method=douglas_reduction threshold=0 reduction=50 type=line where="cat<3"		v.generalize -c input=roads output=roads_douglas_reduction2 method=douglas_reduction threshold=0 reduction=50 type=line layer=1 where="cat<3"
	</pre>		</pre>

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	<pre>		<pre>
	v.generalize input=roads output=roads_displacement2 method=displacement \		v.generalize input=roads output=roads_displacement2 method=displacement \
	threshold=75 alpha=0.01 beta=0.01 iterations=20 cats=1		threshold=75 alpha=0.01 beta=0.01 iterations=20 layer=1 cats=1
	</pre>		</pre>

Neteler at 22:22, 27 April 2014

2014-04-27T22:22:59Z

← Older revision		Revision as of 22:22, 27 April 2014
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	== References ==		== References ==


	TODO		TODO

			== See also ==

			Use cases:
			* http://www.custom-scenery.org/Line-data-simpl.275.0.html


	== AUTHORS ==		== AUTHORS ==

Neteler: link to MAN

2014-04-27T22:18:55Z

link to MAN

← Older revision		Revision as of 22:18, 27 April 2014
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	== MODULE ==		== MODULE ==
	~~'''''~~v.generalize~~'''''~~
			* GRASS 6.4 {{cmd\|v.generalize}}
			* GRASS 7.x {{cmd\|v.generalize\|version=70}}

	== TUTORIAL ==		== TUTORIAL ==

Neteler: /* AUTHORS */ fix cat

2013-06-16T15:12:40Z

AUTHORS: fix cat

← Older revision		Revision as of 15:12, 16 June 2013
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	[[Category: Documentation]]		[[Category: Documentation]]
	[[Category: Tutorial]]		[[Category: Tutorial]]
	[[Category: ~~Generalization operators~~]]		[[Category: Vector]]

⚠️NikosA: added Category: Generalization operators

2013-01-12T20:39:24Z

added Category: Generalization operators

← Older revision		Revision as of 20:39, 12 January 2013
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	[[Category: Documentation]]		[[Category: Documentation]]
	[[Category: Tutorial]]		[[Category: Tutorial]]
			[[Category: Generalization operators]]

⚠️HamishBowman: /* Simplification */ -r no longer works

2013-01-12T10:44:25Z

Simplification: -r no longer works

← Older revision		Revision as of 10:44, 12 January 2013
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	we obtain a map with only 1726 vertices. A disadvantage of the command above is that		we obtain a map with only 1726 vertices. A disadvantage of the command above is that
	it never removes the lines. If we also want to remove the small lines, we need to run		it never removes the lines. <strike>If we also want to remove the small lines, we need to run
	the command above with the -r flag. If the -r flag is presented, lines shorter than		the command above with the -r flag. If the -r flag is presented, lines shorter than
	threshold and areas with areas less than threshold are removed:		threshold and areas with areas less than threshold are removed:
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	v.generalize -r input=roads output=roads_douglas method=douglas threshold=200 --overwrite		v.generalize -r input=roads output=roads_douglas method=douglas threshold=200 --overwrite
	</pre>		</pre>
			</strike>
			Fixme: -r flag and method=remove_small have been removed. Try v.clean for removing small areas or
			v.edit map=mapname type=line tool=delete query=length thresh=0,0,-0.5
			<strike>
	In this case, roads_douglas has only 850 vertices and it contains 387 lines whereas		In this case, roads_douglas has only 850 vertices and it contains 387 lines whereas
	the original map (roads) contains 825 lines. In this case, the output map has very few details,		the original map (roads) contains 825 lines. In this case, the output map has very few details,
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	[[Image:v.generalize.pic2.png\|center\|thumb\|400px\|caption text here]]		[[Image:v.generalize.pic2.png\|center\|thumb\|400px\|caption text here]]
			</strike>
	It is also possible remove small lines/areas only (without any simplification). This is achieved		<strike>It is also possible remove small lines/areas only (without any simplification). This is achieved
	by method=remove_small:		by method=remove_small:

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	[[Image:v.generalize.pic3.png\|center\|thumb\|400px\|caption text here]]		[[Image:v.generalize.pic3.png\|center\|thumb\|400px\|caption text here]]
			</strike>

	Douglas-Peucker Algorithm has very reasonable results, but it is very hard to find(guess) the		Douglas-Peucker Algorithm has very reasonable results, but it is very hard to find(guess) the

⚠️HamishBowman: /* Smoothing */ spelling and caption

2013-01-12T09:26:18Z

Smoothing: spelling and caption

← Older revision		Revision as of 09:26, 12 January 2013
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	Note, that a difference between "Chaiken" and "Hermite" is that the lines produced		Note, that a difference between "Chaiken" and "Hermite" is that the lines produced
	by "Chaiken" "inscribe" the ~~orginal~~ lines whereas the "Hermite" lines "circumscribe" the		by "Chaiken" "inscribe" the original lines whereas the "Hermite" lines "circumscribe" the
	original lines as can be seen in the picture below. (Black line is original line,		original lines as can be seen in the picture below. (Black line is original line,
	green line is "Chaiken" and blue is "Hermite")		green line is "Chaiken" and blue is "Hermite")
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	</pre>		</pre>

	[[Image:v.generalize.pic13.png\|center\|thumb\|400px\|~~caption text here~~]]		[[Image:v.generalize.pic13.png\|center\|thumb\|400px\|method=distance_weighting]]

	Also, very good resutls can be obtained by the "Snakes" algorithm. On the other hand,		Also, very good resutls can be obtained by the "Snakes" algorithm. On the other hand,
	it is the (~~assymtotically~~) slowest		it is the (asymptotically) slowest
	smoothing algorithm implemented in this however. Behaviour of this algorithm is controlled by		smoothing algorithm implemented in this however. Behaviour of this algorithm is controlled by
	"alpha" and "beta" parameter. Reasonable range of values for these two parameters is [0..5]		"alpha" and "beta" parameter. Reasonable range of values for these two parameters is [0..5]

Neteler: /* AUTHORS */

2011-12-04T11:40:10Z

AUTHORS

← Older revision		Revision as of 11:40, 4 December 2011
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	* Wolf Bergenheim, Mentor		* Wolf Bergenheim, Mentor

	[[Category:Documentation]]		[[Category: Documentation]]
			[[Category: Tutorial]]